Conventional charge balance converter ADC's provide a single bit output with gain ranging. In order to achieve gain matching the single bit quantizer output is submitted to a gain multiplier which sets the gain range and delivers it to a thermometer decoder whose output then operates the feedback elements, e.g. capacitors. ΣΔ capacitance-to-digital converters (CDC) are a type sigma delta modulator ADC with a modification to convert an unknown capacitance e.g. a capacitance pressure sensor output to a digital output code. Aside from this modification, they share typical characteristics of sigma delta modulators e.g. over sampling, negative feedback, quantization noise shaping etc. CDC's can achieve multiple input gain ranges by selecting a subset of multiple feedback unit elements from a larger array for the duration of a conversion. These unit elements have minor mismatching between them due to process manufacturing imperfections. This mismatch gives rise to gain errors across the input ranges of the CDC, as the user changes input range. Gain error can be reduced by calibrating for each input range with a known calibration standard value during final test. For multiple input gain ranges, multiple standard values are required, increasing the cost of final test both in time to calibrate across all ranges and increased complexity/hardware. Also, the gain calibration does not hold for gain drifts with time and temperature.
Another type of charge balance converter ADC, provides a multi-bit output directly to a thermometer decoder, without any independent gain setting e.g., U.S. Pat. No. 5,406,283, entitled MULTIBIT OVERSAMPLED DAC WITH DYNAMIC ELEMENT MATCHING, Leung et al., Apr. 11, 1995 hereby incorporated in its entirety by this reference. The decoder output is delivered to a dynamic element matching algorithm (DEM) which patterns the switching signals to the feedback elements to improve linearity by minimizing the in-band errors embodied by the mismatch of these elements.